Composite metal insert



0d. 1950 w. E. KINGSTON 2, ,263

COMPOSITE METAL INSERT Filed March 4, 1944 r0 .4 i4! BALANCE F6 INVENTOR Patented Oct. 3, 1950 UNITED STATES PATENT orrlca COMPOSITE METAL INSERT Walter E. Kingston, Emporium, Pa, uaignor to Sylvania Electric Products Inc Emporium, 2a., a corporation of Massachusetts Application March 4, 1944, Serial No. 525,124

4 Claims. (Cl. -1903) This invention relates to composite wires and metal strips, which can be sealed into soft glass in a vacuum-tight manner, and in particular to a copper clad wire or metal strip having a core of iron-nickel-chromium alloy.

It is known in the art of sealing-in leading-in conductors for incandescent lamps, gas and vapor discharge lamps, mercury rectifiers, radio tubes and similar discharge devices, to use leading-in conductors whose average expansion coeflicient matches, at least for many practical purposes within a certain temperature range, that of the glass into which the conductors are sealed, so as to insure a workable vacuum-tight seal. A wire material has been in use, since as early as 1915, sold under the trade name of Dumet', which consists of a nickel-iron core of an expansion coefficient less than that of the cooperating "glass, sheathed with a copper sleeve.

This prior known material was, and still is.

widely used for making stems for incandescent lamps, radio tubes and similar devices, and is quite satisfactory for its purpose, but is limited to glass seals with wires having a maximum diameter of .025" to .030". No satisfactory vacuum-tight metal to glass seals are obtainable with Dumet wires of diameters larger than .030".

Some of the more important reasons for this limitation are as follows:

1. While it is possible to obtain a "radial average expansion of the copper sheathed nickeliron wire to match that of glass, the expansion of the core alloy in the longitudinal direction does. of course, not take part in this averaging effect.

2. The soft copper sheath which must make up 'the difference in heat expansion between the alloy core and the surrounding glass must always be kept under radial compression in order to avoid a tearing of the bonds between core, copper sheath and glass, within the limits of the endurable stresses. This condition cannot be fulfilled for larger diameters over the entire requisite temperature range, because the inflexion points of the expansion curves of the three materials are not located at the same temperature. As long as the diameter of the wire is small, the maximum tensions occurring at the most unfavorable temperatures met in practical work are not exceeded.

3. The ratio of the radius of the core to the thickness of the copper sheath must be properly chosen so as to result in a satisfactory overall matching of the radial expansion of the sheathed core and the glass to which it is sealed. In the case of the commercial Dumet wire, the copper thickness is about 12% of the wire core. This ratio of copper thickness to alloy core leads to excessive stresses in the glass seal for wire diameters above about .030

From a mere viewpoint of matching expansion coemcients, the use of sheathed wires in vacuumtight metal-to-glass seals can today be completely avoided, if desired. Alloys have been developed which exactly match the expansion coefllolents of the most popular glasses, over a very wide temperature range, and which have surface properties insuring a very high vacuum-tight coherence between metal and glass. One of the most popular alloys of this type is known under the trade name Alloy #4, as disclosed in U. S. Patent No. 2,284,151, and consists substantially of 42% Ni, 6% Cr, 0.1% A1, rest iron with the exception of small amounts of de-oxidizers and/or casting fluxes.

In ultra-high frequency work itis, however, frequently required to reduce the electrical surface resistance of the sealing-in leads, and it is, therefore, desirable in this case to provide leads with a surface coating of higher conductivity than that of Alloy #4. The higher the frequency of the operating currents, the thinner may be the high conductivity coating on a lowconductivity core metal lead, as is well known, due to the decreasing depth of penetration of electric current on the surface of a conductor with increasing frequency (skin-effect) In my research work on developing such a ultra-high frequency lead-in sealing conductor, I was led by these considerations, and started with applying a thin electrolytic copper plating to the surface of a wire or ribbon made of Alloy #4 in order to obtain the desired high conductivity surface, and in order to overcome the deficiencies of "Dumet wire for larger diameters. Such a copper coating must evidently be thin enough not to materially change the radial overall heat expansion of the coated Alloy #4 from that of the uncoated alloy.

Because of the metallurgical structure or the "Alloy #4," electroplating was not, however, successiul. Prior to this invention, no simple and satisfactory production method of electroplating Alloy #4 with copper has been developed whereby a uniform, well-conducting copper coating of the proper thickness and density can be obtained.

I have, however, discovered that other methods can be used which yield the desired high conductivity surface coating on Alloy #4, which does not materially change the overall radial expansion coefllcient of a sheathed "Alloy #4 lead wire or strip as compared to those of the straight Alloy #4 lead-in conductors.

It is, therefore, a principal object of the invention to provide a composite metal wire or strip whose expansion coefficient is materially the same as that of Alloy #4, and has a vacuum tight, tenaciously bonded surface coating of high electrical conductivity.

Another principal object of the invention is to provide means for producing a copper sheathed nickel-chromium-iron alloy wire or strip, in which the thickness of the copper coating is only about 3% of the thickness of the wire or strip of the core material.

A further object of the invention is to provide means for sheathing a copper cylinder over a chromium-nickel-iron core in a vacuum-tight manner.

A still further object of the invention is to provide a vacuum-tight lead-in metal-to-glass seal, which yields maximum conductivity for ultrahigh frequency currents.

A feature of the invention relates to a specially prepared copper clad alloy metal insert for airtight sealing to lead glasses or other soft glasses, as distinguished from the well-known hard glasses.

The accompanying drawing illustrates a preferred embodiment of a composite metal insert of this invention.

The insert here shown has a thickness greater than .030 inch and comprises a core of an alloy containing approximately 38 to 44% nickel, 3 to 8% chromium and .1 to .4% aluminum the balance being substantially entirely iron, the core having a sheath of copper whosethickness is .009 inch.

According to one embodiment of the invention, I use a core of Alloy #4" (e. g., the one consisting of approximately 42% Ni, 3 to 8% Cr, 0.1% Al, balance substantially entirely iron and free from cobalt). The core may be a rod of a diameter of about 1", or whatever may be convenient, and of sufficient length to be drawn into large length of smaller diameter wire. Over this core is placed a copper sheath (hollow cylinder) of a cross sectional area of approximately to 25% of the core cross section and preferably 10%. Between the copper sheath and the core is placed a thin sheet of brass, or a 70-30 silver-copper alloy whose melting point is somewhat below that of the copper. The surface of the core alloy must be clean at all times, to prevent oxidation of the surface of the core metal. Alloy #4 very easily oxidizes and the formation of CrzOs must be avoided, so as to insure the proper alloying of the copper-brass-#4 alloy surfaces during the brazing. This objective can be attained by brazing in dry hydrogen at a temperature slightly above the melting point of brass but we l below that of copper and that of Alloy #4. The brazed bar is then swaged and cold drawn to the desired wire diameter. The finished wire is cleaned and borated to inhibit oxidation of the copper surface. The percentage of copper and Alloy #4 remains substantially unchanged during the drawing operation.

The result is a composite material whose heat expansion characteristics are nearly enough equal to that of Alloy #4 as to produce excellent air-tight glass seals for wire from the lowest desired diameter up to .070" and more. The high conductivity of the copper sheath insures the desired low losses for operation at ultra-high frequencies. The density of the copper sheath obtained by the cold rolling, swaging and wire drawing is so high that the resulting ultrafrequency resistance of the wire is for all practical purposes equal to that of pure copper.

In another embodiment of my invention, I proceed substantially in the manner described, but the swaged ingots are cold rolled rather than drawn into wire, whereby a flat ribbon-like leadin conductor is obtained which is preferable to the wire in certain high frequency applications. If desired, the core may be of fiat "Alloy #4" strip, copper-clad on one or both sides before cold rolling or swaging.

What I claim is:

1. A lead-in conductor for an electron discharge tube comprising a flat metal ribbon including a core ribbon having a thickness greater than .030 inch made of 42% Ni, 6% Cr, 0.1% Al, rest iron, and a copper sheath brazed over the core, covering at least the flat ribbon surfaces of the core, the thickness of each brazed-on copper layer being less than 3% of the thickness of the core ribbon.

2. A composite metal insert having a thickness greater than .030 inch for scaling to vitreous articles and the like comprising a core of an alloy containing approximately 38 to 44% nickel, 3 to 8% chromium, and .l to .4% aluminum, the balance being substantially entirely iron, and a sheath of copper whose thickness is approximately 3% of the thickness of said insert tenaciously bonded to the core for the purposes set forth.

3. A composite metal insert having a thickness greater than .030 inch for sealing to soft glass and the like comprising a core of an iron base alloy containing approximately 42% nickel, approximately 6% chromium and .1 to .4% of a metal selected from the group consisting of aluminum, zirconium, calcium, the balance being substantially entirely iron, said core having a surface sheath of copper which is brazed to the core ina vacuum-tight manner, said copper sheath having the thickness approximately 3% of said metal insert.

4. A composite metal insert accordin to claim 3 in which the surface sheath is in the form of a thin walled tubular member brazed to the core.

WALTER E. KINGSTON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,649,094 Brace Nov. 15, 1927 1,695,791 Yunck Dec. 18, 1928 1,696,791 Yunck Dec. 18, 1928 1,872,354 Scott Aug. 16, 1932 1,989,236 Laise Jan. 29, 1935 2,125,858 Hood Aug. 2, 1938 2,163,407 Pulfrich June 20, 1939 2,219,438 Beeson Oct. 29, 1940 2,259,312 Lee Oct. 14, 1941 2,284,151 Kingston May 28, 1942 

2. A COMPOSITE METAL INSERT HAVING A THICKNESS GREATER THAN .030 INCH FOR SEALING TO VITREOUS ARTICLES AND THE LIKE COMPRISING A CORE OF AN ALLOY CONTAINING APPROXIMATELY 38 TO 44% NICKEL, 3 TO 8% CHROMIUM, AND .1 TO .4% ALUMINUM, THE BALANCE BEING SUBSTANTIALLY ENTIRELY IRON, AND A SHEATH OF COPPER WHOSE THICKNESS IS APPROXIMATELY 3% OF THE THICKNESS OF SAID INSERT TENACIOUSLY BONDED TO THE CORE FOR THE PURPOSES SET FORTH. 